1. Field of the Invention
This invention relates to closed-cycle mechanical cooling systems used with thermal analysis instruments such as differential scanning calorimeters ("DSCs") to control the temperature of a sample undergoing analysis.
2. Background
Mechanical Cooling Accessories (MCAs) have been sold for many years, for use with Differential Scanning Calorimeters. The MCAs allow operation of DSCs in the temperature range of -70.degree. C. to 350.degree. C. The MCAs are economical and convenient to use because they do not use expendable coolants such as liquid nitrogen, as do other types of cooling systems for DSCs. However, prior art MCAs have not been very successful commercially, because they cause a large increase in baseline heat flow noise. Prior art MCAs also suffer from problems caused by condensation and subsequent frost build-up on the exterior of the MCA's cooling head assembly.
An example of a cooling head for a prior art MCA is shown in FIG. 1. The prior an MCA is a two stage vapor compression refrigeration system. It is specially designed to mate with a DSC cell. The cooling head comprises a hollow copper cylinder 1, with a hollow stainless steel cylinder 2 brazed to the top of copper cylinder 1. Stainless steel cylinder 2 has an annular cavity 3. Liquid refrigerant (such as ethane) is fed into this cavity where it evaporates, thus extracting heat from the evaporator formed by copper cylinder 1 and stainless steel cylinder 2. Copper cylinder 1 is in direct physical and thermal contact with the top surface 4 of the DSC cell. Heat flows from the top of the cell to blocks 1 and 2, and is removed by the evaporating refrigerant. One or two insulating rings 5 of polyamide plastic, approximately 0.005" thick are inserted in the heat flow path between the DSC cell and copper cylinder 1 to limit the heat flow from the DSC cell. Insulating rings 5 are necessary to reduce the cooling effect such that the DSC cell can reach a temperature of 350.degree. C., and to reduce thermal noise in the DSC heat flow signal. The evaporator is enclosed in a housing 6 which is filled with thermal insulation 7.
Thermal noise in the DSC heat flow signal is produced by fluctuations in the evaporator temperature, caused by evaporation of the liquid refrigerant. In the prior art MCA, the fluctuations in the evaporator temperature are very irregular. The fluctuations cause changes in the heating rate of the DSC cell, which causes variations in heat flow to and from the DSC cell, thus producing noise in the heat flow signal.